1. Field of the Invention
The present invention relates to an electromagnetic driving device in which a movable member having a body to be driven with respect to a side body is provided slidably in an axial direction. More specifically, the invention relates to an electromagnetic driving device suitable for being used for a driving mechanism which drives a movable lens for focusing and a movable lens for zooming in a lens mirror cylinder mounted in a video camera and the like.
2. Description of the Prior Art
In a lens mirror cylinder of a video camera and the like which has an autofocus function and an electric zoom function, in general, there is provided a driving means for moving a movable lens for focusing and a movable lens for zooming in a direction of the optical axis. As a driving means of this kind, for example, an electromagnetic driving-system actuator having a coil and a magnet has been used relatively in a wide range.
The lens mirror cylinder which uses the aforementioned electromagnetic driving-system actuator is composed of an inner-focus unit which comprises a front ball unit, a variator forming a zoom unit, a diaphragm device, a fixed lens and a movable lens group, provided in the outer housing in order from the front of the camera.
The aforementioned variator is to determine the Wide end and the Tele end of the camera lens system, and the moving range thereof is wide. Therefore, as a driving mechanism of this variator, there can be used mainly a stepping motor, and a rotation-linear movement conversion mechanism which converts a rotation movement to a linear movement of the stepping motor.
The movable lens of the inner-focus unit is a lens which moves to image a subject on a face of a camera tube of, for example, a CCD solid-state image sensing device settled at the back of the lens mirror cylinder, and the moving range thereof is narrower than that of said variator.
Therefore, as a driving mechanism for moving the movable lens in the inner-focus unit, there can be used an electromagnetic driving device such as a voice coil motor which is suitable for moving in a narrow range.
Conventional electromagnetic driving device is composed of, as shown in FIG. 16, a fixed member 102 which is housed and fixed in the outer housing 101, and a movable member 103 which is slidably movable in an axial direction to the fixed member 102, and two axes 104a and 104b are arranged along the optical axis in the inside of said outer housing 101. Furthermore, the fixed member 102 is composed of a yoke frame 105 and a magnet 106 fixed to this yoke frame 105.
Concretely, said yoke frame body 105 is arranged backward and composed of a rectangular connecting board 108 in which a rectangular hole 107 is formed in the central thereof, four outer frame yokes 109 formed on all sides of the end faces of this connecting board 108 toward the forward direction of the axis, and an inner frame yoke 110 in an angular tube-like shape which stands up from the periphery of the hole of said connecting board 108 to forward direction in the axial direction, which are integrally connected. A magnet 106 is bonded to the inner face of the outer frame yoke 109, respectively, with an adhesive and the like, and is singly magnetized so that the magnetic pole differs in the direction of from the outer frame yoke 109 toward the inner frame yoke 110.
The movable member 103 is composed of a plastic frame body 112 in which a focus lens 111 is attached and a bobbin 113 bonded to the back face of the frame body 112.
In the plastic frame body 112, the outer shape is formed in a rectangular form, and a frame body 114 in which a hole having the same diameter with that of the focus lens 111 is provided in the central part thereof, a cylindrical lens-holding cylinder 115 which projects from the periphery of the hole of this frame body 114 forward to fix and hold said focus lens 111, and bearing portions 117a and 117b projected outward from the side face of this lens-holding cylinder 115 having insertion holes 116a and 116b provided therein, through which the axes 4a and 4b of the outer housing 101 are inserted, in the vicinity of the end portion thereof are integrally formed therein.
The bobbin 113 is formed in an angular tube-like shape by a synthetic resin, and has a hollow portion (not shown) in the central portion thereof, whose cross section is a rectangular shape. In the bobbin 113, there is formed a winding groove in the backward portion of the side face, and a coil 118 is wound to the winding groove in an axial direction. In the bobbin 113, the size of the hollow portion is formed to be larger than that of the outer shape of the inner yoke 110 in said fixed member 102, and the size of the outer shape thereof is formed to be smaller than that of a rectangle formed by a line which connects the end face of four magnets bonded to the outer frame yoke 109, thereby it can be housed in the space formed in the opposite faces of the inner frame yoke 110 of said fixed member 102 and the four magnets 106 bonded to the outer frame yoke 109.
When the bobbin 113 in said movable member 103 is housed in said space, the coil 118 wound to the bobbin 113 is located between the inner frame yoke 110 and the magnet 106. At this time, a closed magnetic circuit which is a flux path through which the magnetic flux from the magnet passes, that is, a closed magnetic circuit in which said magnetic flux passes through magnet 106--inner frame yoke 110--connecting board 108--outer frame yoke 109--magnet 106 is formed by the magnet 106 and the yoke frame body 105, and the coil 118 of the bobbin 113 is arranged on such a closed magnetic circuit.
Accordingly, when a drive current is supplied to the coil 118, a magnetic flux is caused in the direction corresponding to the direction of the drive current, thereby a moving force forward or backward is given to the coil 118, and by this moving force, the movable member 103 is moved to the fixed member 102.
In said conventional electromagnetic driving device, a body to be detected, such as a magnet, is generally bonded to the fixed member 102 and a sensor for detecting, for example magnetically, said body to be detected is provided on the movable member 103, in order to detect the relative position of the movable member 103.
In this case, it is required to wire to the sensor provided on the movable member 103, however, since the movable member 103 moves regularly toward the fixed member 102, there are caused such problems that the moving range is restricted by the wiring, or that the moving speed is decreased by the load imposed on the wiring.
Furthermore, since the magnet 106 is singly magnetized so that the magnetic pole differs in the direction of from the outer frame yoke 109 toward the inner frame yoke 110, the magnetic flux of the closed magnetic circuit formed with the inner frame yoke 110, the outer frame yoke 109 and the magnet 106 is concentrated on one or two spots. Thereby, the yoke thickness of the yoke frame body 105 has to be increased in proportion to the movable distance of the movable member 103, and the moving distance of the movable member 103 is increased. Therefore, there is a problem in that when it is a long stroke, the weight of the yoke frame body 105 increases, and the electromagnetic driving device becomes heavy.
Furthermore, in the conventional electromagnetic driving device, a plastic frame body 112 which holds the focus lens 111 closes the front of the bobbin 113 having a hollow portion, and the inner frame yoke 110 of the fixed member 102 is also formed in an angular tube-like shape, thereby the maximum moving range of this movable member 103 is prescribed by the length in the axial direction of the hollow portion of the bobbin 113, particularly, by the length in the axial direction excluding the length of the width of the winding groove.
Therefore, in order to make the movable member 103 slidable against the fixed member 102, that is, to make the focusing possible by an electromagnetic driving system, it is required to make the length in the axial direction of the bobbin 113 large, and to provide a relief h equivalent to the moving distance in addition to said winding groove. In this case, the length which puts together the fixed member 102 and the movable member 103, especially when the movable member 103 is moved forward, becomes the length which adds up the length of the lens-holding cylinder 115 which holds the focus lens 111, the length of the bobbin 113, and the length of the fixed member 102, thereby the length thereof becomes very large. Therefore, in the conventional electromagnetic driving device, there is a problem in that the length in the axial direction of the lens mirror cylinder becomes large to make the overall size of the camera large.
Furthermore, due to the above structure, the end face of the inner frame yoke 110 and the end face of the outer frame yoke 109 in the fixed member 102 has to be open, thereby the magnetic combination of the inner frame yoke 110 and the outer frame yoke 109 becomes one way only. Therefore, in order to improve the magnetic efficiency, the thickness of the inner frame yoke 110 and the outer frame yoke 109 must be increased, thereby there is a problem in that the weight of the electromagnetic driving device, and the weight of the whole camera becomes large.
The above problem becomes more conspicuous with the increase of the moving distance of the movable member 103, and the improvement of these problems has been desired.